Recently, semiconductor lasers have become of increasing importance because of their possible use as a signal source in optical-fiber telecommunications systems.
These devices consist of a crystal composed of layers of semiconductor material having different characteristics of doping and electrical conductivity. Such a structure, which electrically performs as a p-n junction, from the optical standpoint supplies a waveguide for an optical field which is generated inside the device by the laser effect. This phenomenon takes place thanks to a convenient choice of the characteristics of the above-cited semiconductor layers and by a direct device biasing.
Under these conditions a stimulated recombination is obtained of electrons and holes injected into the crystal near the main device junction. In this way the optical field generated in the laser resonator by spontaneous emission, interacting with charge carriers, is amplified while propagating in the crystal waveguide and is partly transmitted to the exterior through the end faces of the crystal.
Originally, laser devices were, from the electrical standpoint, simple p-n junctions between semiconductors having equal gaps of forbidden energy (i.e. so called homojunction lasers). These devices had a limited electrical confinement of charge carriers, which diffused into a wide semiconductor zone, and a limited optical confinement in the regions where the laser effect took place.
Consequently, a high threshold current was necessary to reach the laser effect, which could be obtained only at very low temperatures (-200.degree. C.) and in pulsed operation. Moreover, the devices suffered high optical absorption losses.
The device performances were improved by using so-called heterojunctions, i.e. junctions between semiconductors materials having different forbidden energy gaps. These efficiently confine electrons and holes (thanks to high internal barriers of potential inside the crystal band structure), as well as the optical field in the central laser zone, in correspondence with the active layer (thanks to the refractive index profile which is generated in such a structure). Thus electron-photon interaction efficiency is increased, while current threshold is considerably decreased, allowing continuous operation of the laser at room temperature.
Nevertheless, these structures still have disadvantages due to the spatial instability of the laser emission, resulting from the presence of filaments in the active layer. In fact, under these conditions the carrier distribution is easily altered by any irregularity present in the crystal, generating an uneven optical gain profile and still worse, a time varying gain profile. Hence the impossibility of efficiently exploiting the laser emission of such structures.
A further evolution consists of supplying the structure with an additional confinement in the junction plane by the use of strip geometry.
This has been originally obtained by shrinking carrier injection to a strip contact of nearly 10 .mu.m, defined by the deposition of insulating layers on its sides or by laterally damaging the lattice by bombardment with carriers or heavier ions. Which reduces, but does not completely eliminate, conductive and diffusive phenomena affecting carriers in regions near the active layer. The techniques of bombardment in depth (until the active layer is obtained) could further improve the situation, but is difficult to implement.
More efficient confinement methods are obtained by lateral variations by composition (buried heterostructures), in doping (transverse junction structures) and as a result of the thickness of a pair of adjacent layers (rib geometry).
However, from a technological standpoint these methods are difficult to implement. For example, LPE growth (liquid phase epitaxy) is carried out in two steps, with consequent lengthening of fabrication times and increase in the risk of damaging the already grown structure. Other disadvantages concern the growth on non-planar substrates, and generally on surfaces having a different crystalline orientation.